Problem: Which of these numbers is composite? ${3,\ 29,\ 39,\ 43,\ 73}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 3, 29, 43, and 73 each have only two factors. The factors of 39 are 1, 3, 13, and 39. Thus, 39 is the composite number.